1. Technical Field
The present invention relates generally to a lead-free alloy solder composition and more specifically to a tin-silver-copper alloy solder composition.
2. Related Art
A chip carrier may be coupled to a circuit card by a ball grid array (BGA) comprising BGA solder balls. Such BGA solder balls have typically comprised a eutectic alloy composition of 63% tin (Sn) and 37% lead (Pb) which has a low melting temperature of 183xc2x0 C. and is highly reliable. Unfortunately, lead is toxic and environmentally hazardous. As a result, lead-free solders are now beginning to be used commercially. However, many low-melt, lead-free solders have adverse physical characteristics which may cause reliability problems. Thus, there is a need for a reliable low-melt, substantially lead-free solder ball for coupling a chip or chip carrier to the next level of assembly (e.g., coupling a chip carrier to a circuit card).
In first embodiments, the present invention provides a solder composition, comprising a solder alloy,
wherein the alloy is substantially free of lead,
wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu),
wherein the tin has a weight percent concentration in the alloy of at least about 90%,
wherein the silver has a weight percent concentration of X in the alloy,
wherein X is sufficiently small that formation of Ag3Sn plates is substantially suppressed when the alloy in a liquefied state is being solidified by being cooled at to a lower temperature at which the solid Sn phase is nucleated,
wherein the lower temperature corresponds to an undercooling xcex4T relative to the eutectic melting temperature of the alloy, and
wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%.
In second embodiments, the present invention provides a method for forming an electrical structure, comprising:
providing a first substrate and a first solder ball attached to a first electrically conductive pad that is coupled to the first substrate, wherein the first solder ball comprises a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
providing a second substrate and a second electrically conductive pad coupled to the second substrate;
coupling the first solder ball to the second pad;
melting the first solder ball by heating the first solder ball to form a modified solder ball; and
solidifying the modified solder ball by cooling the modified solder ball to a lower temperature at which the solid Sn phase is nucleated, and wherein the lower temperature corresponds to an undercooling xcex4T relative to the eutectic melting temperature of the alloy, wherein the solidified modified solder ball is a solder joint that couples the first substrate to the second substrate, and wherein a silver weight percent concentration X2 in the modified solder ball is sufficiently small that formation of Ag3Sn plates is substantially suppressed during said cooling.
In third embodiments, the present invention provides a method for forming a solder composition, comprising:
providing a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, wherein the silver has a weight percent concentration in the alloy not exceeding about 4.0%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
melting the alloy by heating the alloy; and
solidifying the melted alloy by cooling the melted alloy at a cooling rate that is high enough to substantially suppress Ag3Sn plate formation in the alloy during said cooling.
In fourth embodiments, the present invention provides a method for forming an electrical structure, comprising:
providing a first substrate and a first solder ball attached to a first electrically conductive pad that is coupled to the first substrate, wherein the first solder ball comprises a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, wherein the silver has a weight percent concentration in the alloy not exceeding about 4.0%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
providing a second substrate and a second electrically conductive pad coupled to the second substrate;
coupling the first solder ball to the second pad;
melting the first solder ball by heating the first solder ball to form a modified solder ball; and
solidifying the modified solder ball by cooling the modified solder ball at a cooling rate that is high enough to substantially suppress Ag3Sn plate formation in the modified solder ball during said cooling, wherein the solidified modified solder ball is a solder joint that couples the first substrate to the second substrate.
In fifth embodiments, the present invention provides a pre-soldering electrical structure, comprising:
a first substrate and a first solder ball attached to a first electrically conductive pad that is coupled to the first substrate, wherein the first solder ball comprises a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
a second substrate and a second electrically conductive pad coupled to the second substrate, wherein the first solder ball is coupled to the second pad, wherein the first solder ball is adapted to being melted by being heated to form a modified solder ball, wherein the modified solder ball is adapted to being solidified by being cooled to a lower temperature at which the solid Sn phase is nucleated, wherein the lower temperature corresponds to an undercooling xcex4T relative to the eutectic melting temperature of the alloy, wherein the solidified modified solder ball is a solder joint that couples the first substrate to the second substrate, and wherein a silver weight percent concentration X2 in the modified solder ball is sufficiently small that formation of Ag3Sn plates are substantially suppressed during said cooling.
In sixth embodiments, the present invention provides a post-soldering electrical structure comprising:
a first substrate; and
a second substrate, wherein the first substrate is coupled to the second substrate by a solder joint, wherein the solder joint comprises an alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, wherein the silver has a weight percent concentration in the alloy of X2, wherein X2 is sufficiently small that Ag3Sn plates are substantially absent in the solder joint, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%.
The present invention provides a reliable low-melt, substantially lead-free solder ball for coupling a chip carrier to a circuit card, or for coupling an integrated circuit chip to a chip carrier.